Compared to parental or peer influences, the role of siblings in development has received little attention. Existing research has focused primarily on family constellation variables, such as ordinal position or age spacing. In contrast, the focus of the present project is the qualitative features of sibling relationships--ie. the warmth, conflict, rivalry, and relative power in the relationships. A model is proposed in which such qualitative features are not only expected to be affected by constellation variables, but also by the nature of parent-child relationships, parental management of sibling relationships, and the social and personality characteristics of the children. The project will examine how sibling relationships differ from other relationships, how various sibling relationships differ, and how the characteristics of sibling relationships are related to the other variables in the proposed model. Twelve studies are planned. In the first three, children's perceptions of their relationships with siblings and others are compared with daily records of interactions and the perceptions of family members or friends. Behavioral patterns of interaction with siblings are compared to those with same-age and mixed-age friends in one study and those with mothers and fathers in another study. Two studies will assess changes in sibling and other relationships through major developmental periods of childhood. The associations among sibling relationships, parent-child relationships, parental management of sibling relationships, and the social and personality characteristics of the individual children are examined in a pair of studies. Twin relations and the parenting of twins will also be considered. Finally, continuities and discontinuities in relationships over the course of a life-transition will be examined. Thus, a range of topics will be studied with a number of different methodological approaches. The results that are yielded should lead to a better understanding of the nature of sibling relationships and their role in the social network.